1. Field of the Invention
The present invention relates to a brake system for a vehicle and a method for operating a brake system for a vehicle.
2. Description of the Related Art
Electric and hybrid vehicles have a brake system configured for regenerative braking, with an electric motor operated as a generator during regenerative braking. The electrical energy obtained during regenerative braking is preferably used, following temporary storage, for accelerating the vehicle. In that manner it is possible to reduce a power loss which is exhibited by a conventional vehicle in the case of frequent braking during a drive, and to reduce energy consumption and pollutant emissions of the electric or hybrid vehicle.
However, operation of the electric motor, for example the electric drive motor, as a generator normally requires a certain minimum speed of the vehicle. Therefore, a regenerative braking system is frequently not able to exert a generative braking torque on the wheels of the vehicle for a sufficiently long period before the vehicle traveling in front is at a standstill. For that reason, a hybrid vehicle often has, in addition to the regeneratively operated electric motor, a hydraulic brake system with which it is possible to compensate for the loss in braking effect of the regenerative brake, at least in a low speed range. In that case, even with a full electrical energy store, when the regenerative brake is usually not exerting any braking torque on the wheels, the entire braking torque may be applied via the hydraulic brake system.
On the other hand, in many situations, it is desirable to exert as small as possible a hydraulic braking force on the wheels in order to achieve a high degree of regeneration. For example, following gearshift operations, the uncoupled generator is frequently activated as a regenerative brake to ensure reliable charging of the temporary storage device and a high energy saving.
In general, a driver prefers an overall braking torque of his vehicle corresponding to his actuation of a brake input element, for example his actuation of the brake pedal, irrespective of activation or deactivation of the regenerative brake. Some electric and hybrid vehicles therefore have an automatic system that is intended to adapt the braking torque of the hydraulic brake system to the current braking torque of the regenerative brake in such a way that a desired overall braking torque is maintained. The driver does not, therefore, himself have to assume the function of deceleration controller by adapting the braking torque of the hydraulic brake system to the current braking torque of the regenerative brake by appropriate actuation of the brake input element.
Examples of such an automatic system are brake-by-wire systems, especially EHB systems. Owing to their complex electronics, mechanics and hydraulics, brake-by-wire systems are, however, relatively expensive.
Published German patent document DE 196 51 153 B4 describes a hydraulic brake system that includes four isolating valves, connected downstream of the master brake cylinder, for disconnection of the four wheel brake cylinders during braking by an external force. After disconnection of the four wheel brake cylinders from the master brake cylinder, a wheel brake pressure is said to be adjustable for each wheel individually in each of the four wheel brake cylinders with the aid of a brake assembly composed of a pump drive motor, at least one pump and at least one storage chamber. A generator braking torque of an on-board generator is said to be blendable in that manner. In addition, published German patent document DE 196 51 153 B4 describes a configuration of a master brake cylinder which is said to enable use of the master brake cylinder as a pedal simulator after disconnection of the four wheel brake cylinders.